Motor start winding switch controlled by phase of main winding current

ABSTRACT

In a motor circuit for an AC motor having a main winding and a start winding, a control circuit utilizes a change in phase between the start winding current relative to the phase of another motor circuit signal to remove a trigger signal from a thyristor in series with the start winding in order to remove power from the start winding when the motor reaches a preselected cut-out speed. In one embodiment a change with motor speed of the relative phase between the main winding current and the applied voltage is utilized. Another embodiment utilizes the change with motor speed of the relative phase between the main winding current and the start winding current.

United States Patent [1 1 Woods et al.

[ Dec. 4, 1973 1 MOTOR START WINDING SWITCH CONTROLLED BY PHASE OF MAINWINDING CURRENT [75] Inventors: Richard E. Woods, Markle; William H.Hohman, Bluffton, both of Ind.

[52] US. Cl. 318/221 E, 318/227 [51] Int. Cl. H02p 1/44 [58] Field ofSearch 318/220 R, 221 R,

[56] References Cited UNITED STATES PATENTS 1/1970 Knauer et a1 318/221E 9/1970 Conner 318/221 E 3,671,830 6/1972 Kruperm; 318/221 E PrimaryExaminer--Gene Z. Rubinson Attorney-Axel A. l-lofgren et al.

[ 5 7 ABSTRACT In a motor circuit for an AC motor having a main windingand a start winding, a control circuit utilizes a change in phasebetween the start winding current relative to the phase of another motorcircuit signal to remove a trigger signal from a thyristor in serieswith the start winding in order to remove power from the start windingwhen the motor reaches a preselected cut-out speed. In one embodiment achange with motor speed of the relative phase between the main windingcurrent and the applied voltage is utilized. Another embodiment utilizesthe change with motor speed of the relative phase between the mainwinding current and the start winding current.

35 Claims, 3 Drawing Figures BACKGROUND OF THE INVENTION This inventionrelates to AC motor start winding switches, and in particular, to anelectronic switch which is responsive to changes in relative phasebetween various motor circuit signals that correspond to changes inmotor speed.

Many AC motors have a start winding which provides torque during theinterval from initial application of power until the motor approachessynchronous speed. Once operating speed is reached, however, the torquesupplied by the start winding is no longer needed, and the mostefficient operation of the motor necessitates the removal of power fromthe start winding.

It is known to mount a mechanical centrifugal switch to the motor which,upon reaching the desired speed, opens the start winding circuit.Unfortunately, this approach has all of the disadvantages inherent tomechanical switches (e.g. corrosion, wear, adjustment of moving parts,etc.). Another known approach has been to utilize changes in both therelative phase and magnitude of various motor circuits signals thatcorrespond to changes in motor speed to disconnect the start windingfrom the source of AC power. The disadvantage of this approach lies inthe fact that the relative magnitude between two motor circuit signalsmay vary somewhat independently of changes'in relative phase and thatthe relationship of motor speed to the amplitude of either current orvoltage is dependent upon applied or line voltage which often results inswitching speeds that vary widely with changes in line voltage or otherparameters unrelated to the deisred cut-out speed.

SUMMARY OF THE INVENTION The improved motor start windijng switch of thepresent invention overcomes the disadvantages of the switches notedabove in a novel manner. The switches disclosed herein monitor speeddependent relative phase relationships of a pair of motor circuitsignals independent of the magnitudes of these signals and actuate aswitch to remove power from the start winding when the relative phase ofthe two signals being monitored assumes a value corresponding to apreselected cut-out speed.

In one embodiment of the start winding switch, the characteristic of themotor that the phase difference between the main winding current and theapplied voltage changes with motor speed is utilized. Utilized inanother embodiment is the characteristic of the motor that the phasedifference between the main winding current and the start windingcurrent changes with motor speed. In both embodiments a bidirectionaltriode thyristor in series with the start winding is continuallyprovided with a gating or trigger signal to keep it in low impedancestate until at the cut-out speed a control circuit prevents the triggersignal from being applied. When the trigger signal is removed from thethyristor, the thyristor is disabled from conducting current through thestart winding and thus power is removed.

The control circuit comprises a disable circuit which generates adisable signal each time a selected one of a pair of motor circuitsignals passes through a first preselected phase angle. The triggersignal generator is disabled from supplying the thyristor with triggersignals,

if it receives the disable signal. However, a disable preventing circuitcoupled to the disable circuit is included in the control circuit whichgenerates a disable preventing signal in response to the other of thetwo pair of motor circuit signals passing through a second phase angleto prevent the disable signal from being received by the trigger signalgenerator and thereby maintain power applied to the start winding. Thedisable preventing circuit is effective to prevent disablement so I longas the disable preventing signal occurs prior in time to the disablesignal. When the motor reaches a preselected cut-out speed, the relativephase between the pair of motor circuit signals is such that the disablepreventing signal is not generated prior to generation' of the disablesignal thereby causing the thyristor to lose its trigger signal and toassume a high impedance state to remove power from the start winding.

Thus an important feature of the present invention is the provision ofan electronic start winding switch responsive to the relative phasebetween the main winding current and another motor circuit signal butindependent of the magnitude of the signals to controlpower to the startwinding.

Another feature of the present invention is the provision of anelectronic start winding switch responsive to the relative phase betweenthe main winding current and the applied voltage to control power to thestart winding.

Still another feature of the present invention is the provision of anelectronic start winding switch responsive to the relative phase of themain winding current to the start winding current for controlling powerto the start winding.

A further feature of the present inventionis the provision of anelectronic start winding switch in which a circuit responsive to thephase of one of a pair of motor circuit signals prevents disablement ofa power application controlling switch connected with the start windingby another circuit responsive to the phase of the other of the pair ofmotor circuit signals until reaching a preselected relative phasebetween the pair of signals which corresponds to a preselected motorspeed.

BRIEF DESCRIPTION OF THE DRAWINGS Further features and advantages of theinvention will be apparent from the following description taken inconjunction with the accompanying drawings in which:

FIG. 1 is a circuit schematic of an embodiment of the start windingswitch in which the change with motor speed of the relative phasebetween the main winding current and the applied voltage is utilized tocontrol the application of power to the start winding;

FIG. 2 is a circuit schematic of a different embodiment in which thechange with motor speed of the relative phase between the main windingcurrent and the start winding current is utilized to control theapplication of power to the start winding; and

FIG. 3 is a graph of motor speed versus degrees of phase shift relativeto applied voltage of thestart winding current and the main windingcurrent in a typical AC motor.

DESCRIPTION OF THE PREFERRED EMBODIMENT Turning now to the drawings, aschematic circuit of a first embodiment of the present invention isshown in FIG. 1, and includes an AC electric motor having a main winding20 and a start winding 22 both connectable with a suitable source of aperiodic voltage 24, such as a line 115 volt AC, 60 Hertz power signal.A motor start winding switch circuit basically comprises a switch meanssuch as a bidirectional triode thyristor 26 connected with start winding22 for causing current to be conducted through start winding 22 exceptwhen disabled by an associated trigger circuit and a control meanscomprising the remainder of the circuitry for disabling the thyristortrigger circuit and thus thyristor 26 when the relative phase betweenthe main winding current and the applied voltage across main winding 20assumes a reselected value corresponding to a preselected cut-out speedof the motor.

The thyristor trigger signal generator provides a gate or trigger signalat gate 28 of the thyristor 26. The trigger signal keeps thyristor 26 ina low impedance state for conducting current through start winding 22until it is removed by the control circuit when the cut-out speed isreached. When the trigger signal is removed, the thyristor assumes ahigh impedance state. The switch control circuit includes a disablemeans which generates a disable signal each time the main windingcurrent passes through a preselected phase angle such as thenegative-to-positive zero cross-over point or zero degree phase angleand a disable preventing means responsive to the applied voltage passingthrough a second preselected phase angle, which may also be the zerocross-over point, to prevent the disable signal from disabling thetrigger signal generator. The disable preventing means is only effectiveto prevent disablement by the disable means when the applied voltagesignal passes through the second phase angle prior to the main windingcurrent passing through the first phase angle during each cycle.

The relative phase between the main winding current and the appliedvoltage varies with motor speed in a typical fashion such as shown inthe graph of FIG. 3. The first and second phase angles are of courseselected such that the second phase angle of the applied voltage occursprior to the first angle of the main winding current only until thedesired cut-out speed is reached. At cut-out speed the relative phasebetween.

the two motor circuit signals has sufficiently changed so that thesecond phase angle of the applied voltage does not occur prior to thefirst phase angle of the main winding current, and thyristor 26 isdisabled to remove power from the start winding.

The trigger signal generator which provides the gating signal at gate 28of thyristor 26 comprises a diode 30 with its anode connected to thejunction between the power source lead at one side 32 of AC power source24 and start winding 22 and its cathode coupled to a junction 34.Junction 34, in turn, is coupledthrough a capacitor 36 to the powersource lead at the other side 38 of AC power source 24, which may be 1grounded, and coupled through a resistor 40 to one side of a DIAC 42,the other side of which is coupled to ground reference potential onpower source lead 38 through a capacitor 44. The other side of DIAC 42is coupled to gate 28 of thyristor 26.

During the positive half-waves of the voltage from power source 24, whenpower source lead 32 is positive with respect to power source lead 38,diode 30 is forward biased and conducts current from lead 32 throughresistor 40 to charge capacitor 44. Early in the positive half-wavecycle capacitor 44 builds up a charge exceedijng the breakover voltageof DIAC 42 to cause it When this occurs the current through diode 30 andresistor 40 again charge capacitor 44 until a sufficient voltage isdeveloped thereacross to again cause DlAC 42 to assume a low impedancestate at which time capacitor 44 is again discharged through DlAC 42 andthe cycle is repeated. Thus,'resist or 40, DIAC 42 and capacitor 44function as a relaxation oscillator to provide periodic trigger pulsesto gate 28 in thyristor 26. The frequency is determined by the chargeand discharge times of capacitor 44.

Also during the positive half-wave of the applied voltage, current isconducted through diode 30 to charge capacitor 36. At the beginning ofthe negative halfwave of the applied voltage, when the voltage on powerline 32 is less than ground reference potential at power line 38, diode30 is reverse biased and capacitor 36 discharges through resistor 40 torepetitively charge capacitor 44 thereby enabling the tigger signal tocom. tinue throughout the negative half-wave. Thus, thyristor 26 iscontinually supplied with gate pulses and thyristor 26 is normallyenabled to conduct current through start winding 22.

As stated, thyristor 26 is disabled from conducting current to startwinding 22 when the gate signals are removed. This occurs when an SCR 46coupled across capacitor 44 is gated into a low impedance state to discharge capacitor 44 thereby preventing it from charging sufficiently toswitch on DIAC 42. SCR 46 may be considered as part of the disablemeans, the'remaining circuitry of which comprises a second bilateraltriode thyristor.48 in series with main winding 20 to conduct mainwinding current therethrough under control of associated gatingcircuitry.

The gating circuitry for thyristor 48 includes a silicon unilateralswitch (SUS) 50 with a gate input 56 coupled through a positivetemperature coefficient (PTC) thermistor S2 and a diode 54 to thejunction between thyristor 48 and main. winding 20. When the positivehalf-wave of the applied voltage exceeds the breakover voltage of SUS50, a portion of the main winding current is conducted throughthermistor S2, diode 54 and SUS 50 to the gate input 56 of thyristor 48which assumes a low impedance state in response thereto to conduct thepositive half-wave of the main winding current. Thyristor 48 remains ina low impedance state throughout the positive half-wave of the mainwinding current until the magnitude of the main winding currenttherethrough falls below a preselected value at which time it switchesto a high impedance state. When this occurs a negative pulse isdeveloped across thyristor 48 which is coupled through a diode 60 and aca-- pacitor 62 to gate input 56 to trigger thyristor 48 innto its lowimpedance state for the negative half-wave of the main winding current.

This negative pulse at the beginning of the negative half-wave of themain winding current occurs only if thyristor 48 conducted the previouspositive half-wave of main winding'current. Thermistor 52 is provided toensure that the thyristor 48 does not conduct the positive half-wave ofthe main winding current in the event of overheating of the motor due toa stalled rotor condition or the like. A suitable location forthermistor S2 is adjacent the inturns of motor start winding 22. Whenthe temperature of the thermistor rises above a preselected temperature,the resistance of the thermistor rises to a corresponding valuesufficiently high to prevent thyristor 48 from turning on during thepositive half-wave of the main winding current. As stated, if thyristor48 does not conduct the positive half-wave of main winding current, thetrigger circuit of diode 60 and capacitor 62 will be ineffective to gateit on during the negative half-wave. Diode 54 in series with SUS 50 isprovided to protect SUS 50 against high reverse voltages thereacross. Aresistor 64 coupled across capacitor 62 is provided to bleed off anycharge thereacross built up during the negative half-wave of the mainwinding current.

Each time thyristor 48 turns off after conducting the negative half-waveof main winding current, a positive pulse is developed across thyristor48 which is coupled through a diode 66 and a resistor 68 to the gateinput 70 of semiconductor controlled SCR rectifier 46. A resistor 72 iscoupled between gate input 70 and ground to provide proper biasing forSCR 46. If this positive disable pulse is received at gate input 70, SCR46 will turn on to disable the trigger signal generator circuit. Thedisable preventing means including an SCR 74 coupled between gate input70 and ground is responsive to the applied voltage passing through asecond preselected phase angle to turn on SCR 74 thereby shunting thepositive pulse disable signal to ground so that it cannot be received atgate input 70 to disable the trigger signal generator. The SCR 74 isswitched on whenever it receives a gate signal at its gate input 76.Gate input 76 is coupled through a resistor 78 to ground for providingproper bias therefor, and coupled to a second SUS 80 which couples agatesignal thereto when the applied voltage passes through the secondphase angle. The other side of SUS 80 is coupled to the power sourcelead 32 through a diode 82, a resistor 84, and a resistor 86, andcoupled to ground through a capacitor 88. The junction between resistors84 and 86 is coupled to ground through an avalanche breakdown diode 88which regulates the voltage threat to limit it to a value equal to itsbreakdown voltage.

During the positive half-wave of applied voltage, current is conductedthrough diode 82, resistor 84, and resistor 86 to charge capacitor 88.At a preselected second phase angle of the applied voltage the chargeacross capacitor 88 is sufficiently high to render SUS 80 conductivewhich discharges capacitor 88 therethrough to gate on SCR 74. The valueof the second preselected phase angle of course depends upon thecharging time of capacitor 88 which, if desired, could be variable. OnceSUS 80 turns on it latches to provide continuous gate current throughoutthe positive halfwave of the applied voltage.

Depending upon the relative phase between the main winding current andthe applied voltage, SCR 74 will turn on either before or after thepositive pulse disable signal. At motor speeds less than cutout speed,SCR 74 will be turned on prior to the positive pulse disable signal andwill shunt it to ground thereby preventing reception by gate 70 of SCR46 and thus preventing disablement of the trigger signal generator.However, at

cut-out speed the positive pulse disable signal will be generated priorto SCR 74 being rendered conductive, and the disable signal will bereceived at gate input to disable the trigger signal generator circuitthereby disabling thyristor 26 to remove power from start winding 22.

A circuit schematic of another embodiment of the present invention isshown in FIG. 2. This circuit is similar to the one shown in FIG. 1except that it compares the phase of the main winding current relativeto the start winding current rather than the phase of the main windingcurrent relative to applied voltage.

A graph of a typical change with motor speed of the relative phasebetween these two signals is shown in FIG. 3. The trigger signalgenerator circuit and the disable signal generator circuit of FIG. 2function in substantially the same manner as the corresponding circuitsin FIG. 1, and the elements of the circuit of FIG. 2 having the same orlike function of corresponding elements in the circuit of FIG. 1 will bedesignated by the same reference numerals.

SCR 74 of the disable preventing circuit gated on to shunt the disablesignal to ground in response to the phase of the applied voltage in thecircuit of FIG. 1, is gated on by a signal responsive to the startwinding current in the circuit of FIG. 2. In particular, when thyristor26 returns to a high impedance state at the end of the negativehalf-wave of start winding current, a

positive pulse is developed there-across. This positive pulse is coupledthrough a diode 90 and a resistor 92 to gate input 76 of SCR 74. Asource of current taken from the junction between resistor 84 andavalanche diode 88 through a resistor 94 is coupled to the anode 96 ofSCR 74 so that once turned on by the gate signal from diode 90 andresistor 92, it will latch into a low impedance state for the remainderof the positive halfwave of the applied voltage. A diode 98 is coupledbetween the anode of SCR 74 and a junction 100, at which appears thepositive pulse disable signal, to isolate gate input 70 of SCR 46 fromthe latching current source for SCR 74. A second diode coupled betweenjunction 100 and gate input 70 of SCR 46 is provided to compensate forthe voltage drop across diode 98.

As similar to the circuit of FIG. 1, when the start winding currentpasses through a second preselected phase angle, such as the zerocross-over point, SCR 74 will be gated into a low impedance state. Ifthis occurs prior to the main winding current passing through the firstphase angle, the positive pulse disable signal will be shunted to groundthereby preventing it from turning on SCR 46 to disable the triggersignal generator. In the circuit of FIG. 2, SCR 74 is gated at thenegativeto-positive zero cross-over point of the start winding current,as is the positive pulse disable signal generated at the zero cross-overpoint of the main winding current. In this situation the cut-out speedwill be that speed at which the phase shift of the start winding currentrelative to the applied voltage is equal to the phase shift of the mainwinding current relative to the applied voltage. As graphicallyillustrated in FIG. 3, this occurs at the speed where the start windingcurrent and main winding current curves cross at 102. By providing adelay circuit to either the positive pulse disable signal or the gatepulse for SCR 74 of the disable preventing circuit, the cut-out speedcould be changed in either direction depending upon which pulse isdelayed.

It should be appreciated that although particular circuit embodimentsare disclosed, many variations could be made while still keeping withinthe broad concept of the present invention, and applicant does notintend to limit the invention to only the particular circuits shown. Forexample, although the disable signal is generated in response to thestart winding current passing through a preselected phase angle andprevented from being effective to disable the thyristor by a circuitresponsive to either the applied voltage or start winding currentpassing through a second preselected phase angle, it is contemplatedthat the disable and disable preventing means could be responsive to thephase of the applied voltage or the start winding current and the mainwinding current, respectively.

I claim:

1. In a motor circuit for an electric motor having a main winding and astart winding both connectable with a source of periodic voltage, saidmotor circuit when connected with the source of periodic voltageproviding a pair of signals including a main winding current signal, therelative phase between which varies with motor speed, a circuit forcontrolling the application of power to the start winding, comprising:

switch means connected with said start winding for causing currentconduction through said start winding except when disabled; and

control means for controlling said switch means including disable meansresponsive to the phase of one of said pair of signals to disable saidswitch means, and

disable preventing means responsive to the phase of the other of saidpair of electrical signals for preventing said disable means fromdisabling the switch means until the relative phase between the pair ofelectrical signals assumes a preselected value corresponding to apreselected cut-out speed of the motor.

2. The control circuit of claim 1 in which said switch means includes aswitch connected with the start winding and trigger means for enablingsaid switch to cause conduction through said start winding.

3. The control circuit of claim 2 in which said disable means comprisesmeans for disabling said trigger means from enabling the switch, saidswitch being disabled when not enabled.

4. The control circuit of claim 2 in which said trigger means generatesa trigger signal and said switch is responsive to said trigger signal tocause current conduction through said start winding until said triggersignal is removed, said trigger signal being removed when the triggermeans is disabled.

5. The control circuit of claim 4 in which said trigger means comprisesa relaxation oscillator.

6. The control circuit of claim 2 in which said switch is in seriescircuit with the start winding.

7. The control circuit of claim 2 in which said switch means comprises athyristor having a gate input connected with the trigger means.

8. The control circuit of claim 1 in which said pair of signals includesthe applied voltage.

9. The control circuit of claim 8 in which said one of said pair ofsignals comprises the main winding current.

10. The control circuit of claim 1 in which said pair of signalsincludes the start winding current.

11. The control circuit of claim 10 in which said one of said pair ofsignals comprises the main winding current.

12. The control circuit of claim 1 in which said one of said pair ofsignals comprises the main winding current.

13. In a motor circuit for an electric motor having a main winding and astart winding both connectable with a source of periodic voltage, saidmotor circuit when connected with the source of periodic voltageproviding a pair of signals including a main winding current signal, therelative phase between which varies with motor speed, a circuit forcontrolling the application of power to the start winding, comprising:

switch means connected with said start winding for causing currentconduction through said start winding except when disabled; and

control means for controlling said switch means including disable meansfor generating a disable signal in response to the said one of said pairof electrical signals passing through a first preselected phase angle,said switch means being disabled in response to reception of saiddisablesignal, and

disable preventing means responsive to the other of said pair ofelectrical signals passing through a second preselected phase angleprior to generation of said disable signal to prevent reception thereofby said switch means, said other of said pair of signals passing throughsaid second preselected phase angle prior to said one of said pair ofsignals passing through said first preselected phase angle only untilthe relative phase between the signals assumes a preselected valuecorresponding to a preselected cut-out speed of the motor.

14. The control circuit of claim 13 in which said switch means includesa switch connected with the start winding and a trigger means forenabling the switch to cause conduction through said start winding, saidtrigger means being disabled in response to reception of said disablesignal.

15. The control circuit of claim 13 in which said first phase anglecomprises zero degrees.

16. The control circuit of claim 13 in which said one of said pair ofelectrical signals comprises the main winding current.

17. The control circuit of claim 16 in which said other of said pair ofsignals comprises the applied voltage.

18. The control curcuit of claim 17 in which said disable preventingmeans includes a switching device means having first and second statesconnecting between said switch means and said disable means, and meansfor switching said switching device means from its first state to itssecond state in response to the applied voltage passing through saidpreselected phase angle, said disable signal being prevented from beingreceived by said switch means when said switching device means is insaid second state.

19. The control circuit of claim 18 in which said second state comprisesa low impedance state and said disable signal is shunted away from saidswitch means through said switching device means when in said lowimpedance state.

20. The control circuit of claim 19 in which said switching device meanscomprises a semiconductor controlled rectifier gatable into a lowimpedance state in response to the applied voltage passing through saidsecond phase angle.

21. The control circuit of claim 16 in which said other of said pair ofsignals comprises the start winding current.

22. The control circuit of claim 21 in which said disable preventingmeans includes a switching device means having first and second statesconnecting between said switch means and said disable means, and meansfor switching said switching device means from its first state to itssecond state in response to the start winding current passing throughsaid preselected phase angle, said disable signal being prevented frombeing received by said switch means when said switching device means isin said second state.

23. The control circuit of claim 22 in which said second state comprisesa low impedance state and said disable signal is shunted away from saidswitch means through said switching device means when in said lowimpedance state.

24. The control circuit of claim 22 in which said switching device meanscomprises a semiconductor controlled rectifier gatable into a lowimpedance state in response to the start winding current passing throughsaid second phase angle.

25. The control circuit of claim 13 in which said disabling meansincludes a switching device means in series with the main winding forconducting main winding current therethrough, said switching devicemeans developing a voltage pulse thereacross at the beginning of eachpositive half-wave of main winding current to produce said disablesignal.

26. The control circuit of claim 25 in which said switching devicecomprises a thyristor.

27. The control circuit of claim 25 in which said switching device meansincludes means response to the temperature of the motor for removingpower from the main winding.

28. The control circuit of claim 13 in which said disable preventingmeans includes 'a switching device means having first and second statesconnecting be-' tween said switch means and said disable means, andmeans for switching said switching device means from its first state toits second state in response to one of said pair of signals passingthrough said preselected phase angle, said disable signal beingprevented from being received by said switch means when said switchingdevice means is in said second state.

29. In an electric motor having a main winding and a start winding bothconnectable with asource of periodic voltage, said motor when connectedwith a source of periodic voltage developing a pair of signalscomprising the main winding current and the start winding current, therelative phase between said pair of signals varying with motor speed, acontrol circuit for controlling the application of power to the startwinding, comprising:

switch means connected with the start winding for causing currenttherethrough except when disabled; and control means for disabling saidswitch means in response to said relative phase assuming a preselectedvalue corresponding to a preselected cut-out speed of the motor. 30. Thecontrol circuit of claim 29 in which said control means includes disablemeans responsive to the phase of one of said pair of signals to disablesaid switch means, and disable preventing means responsive to the 7phase of the other of said pair of electrical signals for preventingsaid disable means from disabling said switch means until said relativephase assumes said preselected value.

31. The control circuit of claim 29 in which said switch means includesa switch connected with the start winding and trigger means forproviding said switch with a trigger signal, said switch being enabledto cause current conduction when said trigger signal is generated anddisabled when said trigger signal is not being generated.

32. The control circuit of claim 31 in which said disable means includesmeans for disabling said trigger' means from generating said triggersignal.

33. The control circuit of claim 30 in which said disable means includesmeans for generating a disable signal in response to one of said pair ofsignals passing through a first preselected phase angle, said switchmeans being disabled in response to reception of said disable signal,and said disable preventing means includes means responsive to the otherof said pair of signals passing through a second'preselected phaseangle. prior to generation of said disable signal to. prevent receptionthereof by said switch means.

34. The control circuit of claim 33 in which said one of said pair ofelectrical signals comprises the main winding current and the other ofsaid pair of signals comprises the start winding current.

35. The control circuit of claim 30 in which said one comprises thestart winding circuit.

1. In a motor circuit for an electric motor having a main winding and astart winding both connectable with a source of periodic voltage, saidmotor circuit when connected with the source of periodic voltageproviding a pair of signals including a main winding current signal, therelative phase between which varies with motor speed, a circuit forcontrolling the application of power to the start winding, comprising:switch means connected with said start winding for causing currentconduction through said start winding except when disabled; and controlmeans for controlling said switch means including disable meansresponsive to the phase of one of said pair of signals to disable saidswitch means, and disable preventing means responsive to the phase ofthe other of said pair of electrical signals for preventing said disablemeans from disabling the switch means until the relative phase betweenthe pair of electrical signals assumes a preselected value correspondingto a preselected cut-out speed of the motor.
 2. The control circuit ofclaim 1 in which said switch means includes a switch connected with thestart winding and trigger means for enabling said switch to causeconduction through said start winding.
 3. The control circuit of claim 2in which said disable means comprises means for disabling said triggermeans from enabling the switch, said switch being disabled when notenabled.
 4. The control circuit of claim 2 in which said trigger meansgenerates a trigger signal and said switch is responsive to said triggersignal to cause current conduction through said start winding until saidtrigger signal is removed, said trigger signal being removed when thetrigger means is disabled.
 5. The control circuit of claim 4 in whichsaid trigger means comprises a relaxation oscillator.
 6. The controlcircuit of claim 2 in which said switch is in series circuit with thestart winding.
 7. The control circuit of claim 2 in which said switchmeans comprises a thyristor having a gate input connected with thetrigger means.
 8. The control circuit of claim 1 in which said pair ofsignals includes the applied voltage.
 9. The control circuit of claim 8in which said one of said pair of signals comprises the main windingcurrent.
 10. The control circuit of claim 1 in which said pair ofsignals includes the start winding current.
 11. The control circuit ofclaim 10 in which said one of said pair of signals comprises the mainwinding current.
 12. The control circuit of claim 1 in which said one ofsaid pair of signals comprises the main winding current.
 13. In a motorcircuit for an electric motor having a main winding and a start windingboth connectable with a source of periodic voltage, said motor circuitwhen connected with the source of periodic voltage providing a pair ofsignals including a main winding current signal, the relative phasebetween which varies with motor speed, a circuit for controlling theapplication of power to the start winding, comprising: switch meansconnected with said start winding for causing current conduction throughsaid start winding except when disabled; and control means forcontrolling said switch means including disable means for generating adisable signal in response to the said one of said pair of electricalsignals passing through a first preselected phase angle, said switchmeans being disabled in response to reception of said disable signal,and disable preventing means responsive to the other of said pair ofelectrical signals passing through a second preselected phase angleprior to generation of said disable signal to prevent reception thereofby said switch means, said other of said pair of signals passing throughsaid second preselected phase angle prior to said one of said pair ofsignals passing through said first preselected phase angle only untilthe relative phase between the signals assumes a preselected valuecorresponding to a preselected cut-out speed of the motor.
 14. Thecontrol circuit of claim 13 in which said switch means includes a switchconnected with the start winding and a trigger means for enabling theswitch to cause conduction through said start winding, said triggermeans being disabled in response to reception of said disable signal.15. The control circuit of claim 13 in which said first phase anglecomprises zero degrees.
 16. The control circuit of claim 13 in whichsaid one of said pair of electrical signals comprises the main windingcurrent.
 17. The control circuit of claim 16 in which said other of saidpair of signals comprises the applied voltage.
 18. The control curcuitof claim 17 in which said disable preventing means includes a switchingdevice means having first and second states connecting between saidswitch means and said disable means, and means for switching saidswitching device means from its first state to its second state inresponse to the applied voltage passing through said preselected phaseangle, said disable signal being prevented from being received by saidswitch means when said switching device means is in said second state.19. The control circuit of claim 18 in which said second state comprisesa low impedance state and said disable signal is shunted away from saidswitch means through said switching device means when in said lowimpedance state.
 20. The control circuit of claim 19 in which saidswitching device means comprises a semiconductor controlled rectifiergatable into a low impedance state in response to the applied voltagepassing through said second phase angle.
 21. The control circuit ofclaim 16 in which said other of said pair of signals comprises the startwinding current.
 22. The control circuit of claim 21 in which saiddisable preventing means includes a switching device means having firstand second states connecting between said switch means and said disablemeans, and means for switching said switching device means from itsfirst state to its second state in response to the start winding currentpassing through said preselected phase angle, said disable signal beingprevented from being received by said switch means when said switchingdevice means is in said second state.
 23. The control circuit of claim22 in which said second state comprises a low impedance state and saiddisable signal is shunted away from said switch means through saidswitching device means when in said low impedance state.
 24. The controlcircuit of claim 22 in which said switching device means comprises asemiconductor controlled rectifier gatable into a low impedance state inresponse to the start winding current passing through said second phaseangle.
 25. The control circuit of claim 13 in which said disabling meansincludes a switching device means in series with the main winding forconducting main winding current therethrough, said switching devicemeans developing a voltage pulse thereacross at the beginning of eachpositive half-wave of main winding current to produce said disablesignal.
 26. The control circuit of claim 25 in which said switchingdevice comprises a thyristor.
 27. The control circuit of claim 25 inwhich said switching device means includes means response to thetemperature of the motor for removing power from the main winding. 28.The control circuit of claim 13 in which said disable preventing meansincludes a switching device means having first and second statesconnecting between said switch means and said disable means, and meansfor switching said switching device means from its first state to itssecond state in response to one of said pair of signals passing throughsaid preselected phase angle, said disable signal being prevented frombeing received by said switch means when said switching device means isin said second state.
 29. In an electric motor having a main winding anda start winding both connectable with a source of periodic voltage, saidmotor when connected with a source of periodic voltage developing a pairof signals comprising the main winding current and the start windingcurrent, the relative phase between said pair of signals varying withmotor speed, a control circuit for controlling the application of powerto the start winding, comprising: switch means connected with the startwinding for causing current therethrough except when disabled; andcontrol means for disabling said switch means in response to saidrelative phase assuming a preselected value corresponding to apreselected cut-out speed of the motor.
 30. The control circuit of claim29 in which said control means includes disable means responsive to thephase of one of said pair of signals to disable said switch means, anddisable preventing means responsive to the phase of the other of saidpair of electrical signals for preventing said disable means fromdisabling said switch means until said relative phase assumes saidpreselected value.
 31. The control circuit of claim 29 in which saidswitch means includes a switch connected with the start winding andtrigger means for providing said switch with a trigger signal, saidswitch being enabled to cause current conduction when said triggersignal is generated and disabled when said trigger signal is not beinggenerated.
 32. The control circuit of claiM 31 in which said disablemeans includes means for disabling said trigger means from generatingsaid trigger signal.
 33. The control circuit of claim 30 in which saiddisable means includes means for generating a disable signal in responseto one of said pair of signals passing through a first preselected phaseangle, said switch means being disabled in response to reception of saiddisable signal, and said disable preventing means includes meansresponsive to the other of said pair of signals passing through a secondpreselected phase angle prior to generation of said disable signal toprevent reception thereof by said switch means.
 34. The control circuitof claim 33 in which said one of said pair of electrical signalscomprises the main winding current and the other of said pair of signalscomprises the start winding current.
 35. The control circuit of claim 30in which said one of said pair of electrical signals comprises the mainwinding current and the other of said pair of signals comprises thestart winding circuit.